A vuilleumier heat pump device is well known in the art such as disclosed in Laying Open unexamined Japanese Patent Application No.1-137164. In the vuilleumier heat pump device, as shown in FIG. 1, a displacer (3H) on hot side (hot side displacer) which is reciprocally movably inserted in a cylinder (1H) on hot side (hot side cylinder) and a displacer (3L) on cold side (cold side cylinder) which is reciprocally movably inserted in a cylinder (1L) on cold side (cold side cylinder) are connected to each other through the medium of a crank shaft (5). Each displacer (3H), (3L) is reciprocated at a set phase difference (e.g., 90.degree.), and volumes of a hot space (9H) and middle-temperature space (10H) each defined and formed in the hot side cylinder (1H) by the hot side displacer (3H) and a cold space (9L) and a middle-temperature space (10L) each defined and formed in the cold side cylinder (1L) by the cold side displacer (3L) are respectively changed. Thereby, an working gas is changed in pressure to form a thermodynamic cycle. Also, heat input at a heater part (14H) which receives heat by a burner (17H) on the hot side cylinder side, heat input from a thermal medium, such as water, at a cooler part (17L) on the cold side cylinder side, and radiation to the thermal medium at middle-temperature heat exchangers (16H), (16L) are respectively conducted so that the thermal medium performs cooling or heating.
A vuilleumier heat pump device disclosed in Laying Open unexamined Japanese Patent Application No.4-240359 changes the cooling capability and heating capability by adjusting flow rate of the thermal medium circulated between the mid-die-temperature heat exchangers (16H), (16L) and the cooler part (17L), and each of indoor and outdoor heat exchangers (23), (25), or by adjusting heating amount or cooling amount.
In the vuilleumier heat pump device, the capability of the cooler part (17L) is controlled by increasing or decreasing a speed of rotation of the crank shaft (5) in such a manner that the crank shaft (5) is driven and rotated by a rotation control motor (21), or by adjusting a combustion rate of the burner (17H).
In this case, however, while the cooling capability and the heating capability are increased according to the increase in the revolution speed, the efficiency is lowered at that time. For example, in case of cooling, when the revolution speed (N) increases as shown in FIG.7, the cooling capability (Qk) is proportionally increased. At the same time, the temperature (Tc) of the working gas in the cold space (9L) lowers as indicated by a solid line in FIG.8 and the temperature (Tm) of the working gas in the middle-temperature spaces (10H), (10L) rises as indicated by a solid line in FIG.9. As a result, the coefficient of performance (COP.sub.L) is gradually lowered as indicated by a solid line in FIG. 10.
This invention has been made in view of the above described disadvantages and has its object of avoiding the lowering of coefficient of performance due to such the capability control in a vuilleumier heat pump device.